A cell-free bacteriocidal substance produced by Streptococcus mutants is lethal for S. pyogenes (group A) and other streptococci. The purified material is destroyed by proteolytic enzymes, is non-dialyzable and does not lyse susceptible streptococci. Studies will be made on the mechanism of action of the bacteriocin and its activity in vivo. BIBLIOGRAPHIC REFERENCES: Linzer, Rosemary, K. Gill, and H.D. Slade. Chemical Composition of Streptococcus mutants Type c Antigen: Comparison to Type a, b, and d Antigens. J. Dental Research 55:A109-A115. 1976. Linzer, Rosemary and H.D. Slade. Characterization of an Anti-Glucosyltransferase Serum Specific for Insoluble Glucan Synthesis by Streptococcus mutans. Infect. Immunity 13:494-500. 1976.